According to a recent tweet from P0sixNinja, the iPhone hacker behind GreenPois0n, an updated version of the jailbreak will be released as soon as he finishes adding the new sandbox patches.
There are good reasons to think that this jailbreak for iOS 4.2.1 will be a tethered jailbreak, thus requiring you to plug your iPhone in and boot it with GreenPois0n every time you shut down your iPhone...